Overview

Brief Summary

There are very few wild flat oysters around anymore. They have become rare due to disease and competition with the Pacific oyster. The oyster shells on the beach are usually remnants from very old animals, dead already for many years. The shell has a rather regular shape, in contrast to the Pacific oyster which can have all kinds of sharp protrusions. The flat oyster is edible. For some people it is only a slimy bunch of salt but, according to oyster-lovers, it is the most delicious thing ever.

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Biology

Native oysters are gregarious animals, and start their lives as males. They mature sexually as males between eight and ten months old. From then on, oysters will change sex regularly, depending on the water temperature. If the temperature reaches 16°C, they become females every three or four years. If the temperature reaches 20°C, they will change to females each year. They only revert to being males during the cooler intervening periods. Oysters may live for as long as 15 years but the usual lifespan is thought to be around six years. Eggs are stored and fertilised in the gill cavity of the female and remain there for a week before becoming free-swimming larvae and being released. The sperm is passed through the gills as part of the normal feeding process. The oyster larvae join the plankton in the open sea until, after 10 or 20 days, they find a surface to attach themselves. Adult oysters feed by filtration, sieving out the plankton using their gills. The towns of Colchester in Essex and Whitstable in Kent have become famous for their Oyster Festivals. Oysters have been an important food source since prehistory, and during the Roman occupation, British oysters were exported in large quantities back to Italy. One claim for eating them is that they act as an aphrodisiac, although there is no scientific proof for this argument.

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Description

The native oyster is a bivalve mollusc, which means 'two shells', and is rough, scaly and yellowish-grey in colour. Each valve differs in shape and size; the left one (the one used by the oyster to attach itself to a surface) is concave, while the right one is flat and fits snugly inside the left. The right valve has concentric rings of a bluish colour, and the whole animal is roughly pear-shaped. Inside the shell, the colours range from blue to grey and include the opalescent 'mother of pearl'. Mother of pearl is secreted by the oyster around any foreign body that gets trapped between the shells, for example, a piece of sand or grit. In time, this builds up and forms a pearl.

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Comprehensive Description

Description

Ostrea edulis is a bivalve mollusc that has an oval or pear-shaped shell with a rough, scaly surface. The two halves (valves) of the shell are different shapes. The left valve is concave and fixed to the substratum, the right being flat and sitting inside the left. The shell is off-white, yellowish or cream in colour with light brown or bluish concentric bands on the right valve. Ostrea edulis grows up to 110 mm long, rarely larger. The inner surfaces are pearly, white or bluish-grey, often with darker blue areas.Also commonly known as the flat oyster and European oyster.

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Distribution

Range

Oysters are found widely around the western European coastline as far north as Spitsbergen, and south to Morocco and the Mediterranean. They can turn up all around the British coast with the best areas being the Thames Estuary, the west coast of Scotland, the Solent, the estuary of the River Fal, and Loch Foyle. They are also cultivated in other parts of the world such as North America, Japan and Australasia.

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Ostrea edulis is associated with highly productive estuarine and shallow coastal water habitats on firm bottoms of mud, rocks, muddy sand, muddy gravel with shells and hard silt. In exploited areas, suitable habitat is/has been created in the form of 'cultch' - broken shells and other hard substrata.

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Threats

Once a plentiful species around the UK coastline, the native oyster was the victim of serious overharvesting during the late 19th century, as they were a staple food of the poor and working classes. In the 15th century, fourpence would buy eight gallons of oysters, but by the beginning of the 20th century, they had become the expensive luxury they are today. This overfishing was probably a consequence of the expanding townships following the Industrial Revolution. Today the principal threat to the wild native oyster comes from disease and two introduced species, the American oyster drill shellfish Urosalpinx cinerea, and the slipper limpet Crepidula fornicata. The disease, Bonamiosis, is spread by a parasitic protozoan Bonamia ostreae. The oyster drill was brought to the UK by accident with imported American oysters and feeds on the young shellfish. The slipper limpet forms dense beds, competing with the oyster for spaces and food resources. It also produces a material known as 'mussel mud', a substance that covers potential oyster beds and makes it difficult for young oysters to establish themselves on a firm surface.

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Management

Conservation

The oyster industry in the UK is a lucrative one, but is now dominated by the introduced Japanese or Pacific oyster Crassostrea giga. However, native oysters are farmed and potential sites are sometimes prepared by dumping broken shells, an aggregate known as 'cultch', which encourages young oysters, called 'spat', to form new beds. The native oyster is listed as a priority species in the UK Biodiversity Action Plan. The shell fishing industry in the UK is carefully regulated, and there is also a closed season on harvesting from 14 May to the 4 August during the critical spawning period, although this does not cover farmed oysters. The principal aims of the UK Action Plan for the native oyster are to maintain the current range of the oyster around the UK coastline and, where possible, increase the population and the number of viable oyster beds. In order to improve the species' chances, a number of laws and directives have been introduced in recent years. In 1987, a ban was imposed preventing the use of TBT-based anti-fouling paints on all vessels less than 25 metres in length. The ban was introduced for these smaller vessels as they are more likely to come into the shallower coastal waters than the larger sea-going ships. Shellfish farmers have welcomed the banning of the use of this paint, which is believed to affect the reproduction rates of oysters. There is also a European Directive governing the spread of diseases prevalent among bivalves. The shellfish industry is being encouraged to conduct more environmental impact assessments in areas thought suitable for re-introduction or, in some cases, on former sites that have become derelict.

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Wikipedia

Ostrea edulis

Ostrea edulis is a species of oyster native to Europe and commonly known as the European flat oyster, Colchester native oyster, mud oyster, or edible oyster (despite this latter name it is not the only oyster that is edible by humans). When mature, O. edulis adults range from 3.8 to 11 centimetres (1.5 to 4.3 in) across.[3] The species naturally ranges along the western and southern coasts of Europe from Norway to Morocco and including most of the British Isles and the Mediterranean coast.[4] Naturally viable populations have appeared in eastern North America from Maine to Rhode Island subsequent to artificial introduction in the 1940s and 1950s.[4]

U.S. oyster growers farm O. edulis in small quantities on both coasts. They are prized for their unique tannic seawater flavor, sometimes described as dry and metallic, and are more expensive than other American oysters.[4] The flavor is considered excellent for eating raw on the half shell.[5][6]